It has been proposed previously to provide seat-belt pretensioners which apply tension to a seat-belt in the event that an accident occurs. Many pretensioners of this type are actuated by a pyrotechnic charge, which generates gas to move a piston within a cylinder, the piston applying tension to a wire which is wound round the spindle of the seat-belt retractor so that seat-belt is wound into the retractor, thus tightening the seat-belt across the lap and the chest of the person wearing the seat-belt. Other pretensioners may move the entire retractor relative to its mounting, or may move an end part of the seat-belt or a guide through which the seat-belt passes, or may move the seat-belt buckle, relative to a mounting. The action of a pretensioner is to ensure that the seat-belt can provide an optimum retaining effecting, thus effectively retaining the person wearing the seat-belt in their seat during the accident situation and minimising the risk of the person wearing the seat-belt moving from their position in their seat to impact with part of the vehicle, such as a steering wheel or dashboard.
Pretensioners of the types described above have been associated with sensors which sense an impact, and have also been associated with sensors which are arranged to predict when an accident is about to occur.
Seat-belt pretensioner arrangements which have been associated with sensors designed to predict when an accident may occur have been associated with sensors of various different types. U.S. Pat. No. 6,394,495, for example, shows a seat-belt pretensioner arrangement in which sensors are provided to sense the braking of the vehicle, the dynamic conditions of the vehicle such as yaw angle speed and yaw angle acceleration and lateral acceleration, and also the acceleration of the vehicle. In the arrangement described an initial or “soft” tensioning of the seat-belt is achieved for excessive yaw angle speed or yaw angle acceleration, and a “hard” tightening profile, with a higher pulling force, can be achieved when other indicating signals exceed the relevant threshold values.
It has been found that the prior proposed arrangements are not sufficiently responsive to evasive manoeuvring. Manoeuvring of this type typically occurs in the brief period of time preceding an accident situation, and the present invention seeks to provide a seat-belt pretensioner arrangement which may be able to pretension a seat-belt in response to evasive manoeuvring of a motor vehicle.